New technologies have been developed to reduce our society's reliance on fossil fuels. For example, electric vehicles have been developed that are operated on battery power. These battery-powered vehicles are becoming increasingly economical, both in purchase price and operational cost, and the battery-powered vehicles are at least partially powered by battery packs. The battery packs generally provide power for propelling the vehicle and powering the various electrical systems used by the driver and other vehicle occupants.
One type of battery-powered vehicle is known as a hybrid vehicle. The hybrid vehicle employs a combustion engine coupled with a combination electric motor-generator in order to provide vehicle locomotion. In some hybrid or mild-hybrid powertrain systems, an electric motor-generator system replaces the conventional starter motor and alternator. When the hybrid vehicle is decelerating or is stopped, the fuel flow to the engine is shut off to improve fuel economy. While the hybrid vehicle is at a standstill, the engine is not turning. The motor-generator system and battery pack of the hybrid vehicle enable this fuel cutoff feature while minimally affecting drivability.
In a strong-hybrid powertrain with an automatic transmission, when the brake pedal is released after a stop, the motor-generator system spins up the engine, and creeps the vehicle forward, similar to a conventional vehicle with an automatic transmission. The engine combustion can be commenced after some period of time, or upon activation of the accelerator pedal. When the driver accelerates, the combustion engine restarts automatically and the hybrid vehicle can be driven in a conventional manner. When the combustion engine is running, the motor-generator acts as a generator to supply the hybrid electric vehicle's electrical power requirements, as well as recharging the battery pack of the vehicle. The vehicle's battery pack and a DC-to-DC converter support the vehicle's electrical loads (fans, radio, etc.) whenever the combustion engine is off.
While the use and adoption of battery powered vehicles, including hybrid vehicles, has numerous advantageous, it can also present a need for additional design considerations, particularly with regard to the battery pack. For example, individual batteries and battery modules formed of one or more batteries, which for the battery pack, generally do not have the same capacity and certain deviations in capacity may occur due to manufacturing and other variances. Considering a normal distribution of capacity for the batteries or battery modules, the weakest battery or battery module in the battery pack may become a limiting factor in the battery pack, thereby decreasing overall efficiency. In an effort to maximize the energy output of the battery pack, some batteries or battery modules may be “overcharged” in order to ensure that the batteries or battery modules have been charged to at least the minimum level of the battery or battery module with the highest capacity, thereby causing an overcharge of those batteries or battery modules with a lower capacity. This overcharging can produce undesirable effects such as decreasing overall battery life.
Similarly, undercharging is undesirable because the battery pack efficiency can be reduced and battery life can also be prematurely shortened. In addition, minor variations in the state-of-charge for the different batteries or battery modules can lead to inefficient energy distribution and to more frequent charging cycles, which also tends to shorten the life of the batteries. These problems, and other similar problems known to those skilled in the art, continue to be the source of operational and maintenance difficulties for battery-power vehicles.
In view of the foregoing, it should be appreciated that it would be desirable to provide battery packs for battery-powered vehicles that address the existing deficiencies previously described and other deficiencies not specifically mentioned in this background of the invention. Furthermore, additional desirable features will become apparent to one skilled in the art from the foregoing background of the invention and following detailed description of a preferred exemplary embodiment and appended claims.